The invention relates to a telephoto zoom lens for 35-mm still camera.
Heretofore, there has been proposed a zoom lens of so-called mechanical correction type comprising a variable magnification system formed by three groups having a positive, a negative and a positive refractive power, respectively, and a relay lens system and in which as the focal length is changed by a movement of the second group along the optical axis during a zooming operation, the third group is moved to correct for a variation in the location of an image plane which is attributable to the movement of the second group.
A high maneuverability is generally required when using a zoom lens of this type, but the available construction is not suitable for portable use in consideration of its weight and size. While various efforts have been made in designing a compact and light zoom lens, by reducing the overall length and reducing the diameter of the front lenses while maintaining a sufficient brightness of the edge of the image field, a variety of difficulties have been encountered with.
In the design of a compact zoom lens, the determination of a distribution of the power among the individual groups is of primary importance. Any lens configuration cannot achieve a satisfactory result without a proper distribution of the power.
To reduce the overall length of the lens, it is essential that the variable magnification system be constructed in a compact manner and that the relay lens system be of a telephoto type.
It is established that it is most effective to assign high powers to the various groups in the variable magnification system while imparting a higher refractive power to the second or the variator lens group to reduce the stroke of movement thereof in order to reduce the size of the variable magnification system. In this instance, the individual surfaces in the second group are obviously given high powers, which must be limited, however, for purpose of correcting the aberrations.
On the other hand, when the relay lens system is formed as a telephoto type, there is a tendency toward a reduced magnitude of the telephoto ratio and a reduced value of Petzval sum. Accordingly, the number of individual lenses must be increased to provide a sufficient correction of a negative Petzval sum which occurs in the variable magnification system. This resulted in a complex and heavy lens arrangement if it is attempted to provide a compact zoom lens by reducing the overall length thereof.